During manufacture of large structures, it is often necessary to accurately align portions of the structure. In the construction of aircraft, many physical alignments are required, often between portions of the airframe that are substantially displaced from each other. The distance between alignment points can vary from several feet to 40 feet or more. Such alignments must be extremely precise and require the rendering of extremely fine lines or points to achieve required tolerances. Notwithstanding the importance of accurate positioning and small size of required alignment marks, it is important to be able to easily remove the marks once the alignment process is completed.
Laser systems have been employed to remotely mark structures. Where fine markings are required, the prior art teaches various focusing techniques so as to enable a laser's beam pattern to be compressed to a fine point. Such focusing is time consuming and is to be avoided if possible. In U.S. Pat. No. 4,148,057 to Jesse, an ultraviolet laser is employed to cure an ink that has been coated on a large surface. The ink (resin) is rolled, brushed or sprayed onto the surface being marked and subsequently illuminated by a laser beam in a predetermined pattern. After the surface is washed, a raised mark is present on the path that the laser beam followed. U.S. Pat. No. 4,547,649 to Butt et al. discloses a laser-heat based system wherein when a surface oxide is heated to approximately 1200.degree. F. The heated portion of the surface oxide returns to the base metal state and contrasts with the remaining surface oxide layer. Such a technique enables superficial marking of zirconium and certain other metals.
U.S. Pat. No. 4,307,047 to Edinger et al. is a further laser-heat based system wherein a plastic object is mixed with another material that upon localized heating, changes color. The laser is controlled to create numbers, letters and other permanent markings thereon.
U.S. Pat. No. 3,657,510 to Rothrock discloses a laser marking system wherein a mask is employed to shape a laser beam to create a desired pattern on the receiving surface. The laser beam causes mechanical changes in the target surface to create the desired mark.
U.S. Pat. No. 4,401,992 to Vorst et al. also discloses a mask for shaping a laser beam. The masked laser beam causes a material containing a dye to decompose and form a color contrast mark on the surface being marked. The material being marked is described as a colored synthetic resin.
U.S. Pat. No. 5,035,983 to Kiyonari et al. describes a composition useful in a laser marking system. The composition contains a non-black inorganic lead compound and a resin. A laser, having a wavelength in the far infrared region, exposes the material and causes the appearance of a black mark. The far infrared laser regime is said to be required due to the heat resistance of the lead compound. U.S. Pat. Nos. 4,769,310 to Gugger et al. and 5,030,551 to Herren et al. both disclose the marking of ceramic materials, glazes, etc. using a laser beam. Both patents describe the rendering of permanent marks in or on a ceramic material or glaze through the use of the laser beam. U.S. Pat. No. 3,747,117 to Fechter employs a laser to expose a metal oxide coatings as to create spots having greater light permeability than the unexposed portions of the oxide. The oxide medium is described as a layer of oxidized aluminum into which a dye solution has been diffused so as to color the oxide. U.S. Pat. No. 4,822,987 to Goldenfield et al. describes a marking method that employs a computer controlled laser to engrave or etch a bar code onto nuclear fuel rods. This process, as well as most if not all of the above noted processes, leaves permanent marks on the surface.
Accordingly, it is an object of this invention to provide an improved laser marking system wherein a surface is marked with non-permanent indicia.
It is another object of this invention to provide an improved laser marking system wherein a fine marking beam pattern is achieved without the requirement of focusing optics.
It is yet another object of this invention to provide an improved laser marking system wherein a surface to be marked may be placed at varying distances from a laser exposure device without any requirement for focusing or refocusing of the laser beam.
It is still another object of this invention to provide a laser marking system wherein a temporary, light responsive medium may be coated on a surface to be marked and later removed from the surface without leaving a marking residue.